1. Technical Field of the Invention
This invention relates to radio telecommunication systems and, more particularly, to a method of selecting either a digital traffic channel or an analog voice channel in a digital cellular telecommunications network.
2. Description of Related Art
In existing radio telecommunications networks such as the Digital Advanced Mobile Phone System (D-AMPS), field measurements often show that performance is inferior to the analog Advanced Mobile Phone System (AMPS) in the areas of connection reliability and voice quality. For example, field tests indicate that a call which is set up on a digital traffic channel (DTC) is more than twice as likely to fail than a call on an analog voice channel (AVC). The handoff failure rate for D-AMPS is also higher than the failure rate for AMPS. In addition, the D-AMPS voice quality is perceived to be worse than AMPS at low signal strength and at high interference levels.
Although there are no known prior art teachings of a solution to the aforementioned deficiency and shortcoming such as that disclosed herein, U.S. Pat. No. 5,355,514 to Borg discusses subject matter that bears some relation to matters discussed herein. Borg discloses a method of determining the transmission quality between a mobile station and its serving base station in a mobile radio system. The combined signal strength of the carrier signal and any disturbing signals is first measured. Then, the signal strength of solely the disturbing signals is measured by taking such measurement while the mobile station and serving base station transmitters are shut down. The quotient between these two signal strengths is a measure of connection quality. The carrier-to-interference ratio (C/I) on the serving channel is thus calculated from the equation: EQU C/I=((C+I)/I)-1.
Thus, Borg discloses a technique to measure the carrier-to-interference ratio (C/I) during a call (i.e., monitor the voice quality). However, Borg does not teach or suggest a method of voice channel reselection in which the expected C/I is calculated before seizing a voice channel to determine whether adequate voice quality will be provided, and reselecting another voice channel if the expected C/I is too low. For an intracell handoff request (i.e., to another voice channel in the same cell), the method of Borg could be utilized to trigger the request. However, Borg does not teach or suggest a method of ensuring that a good voice channel is then selected. In addition, the present invention is triggered by a DTC seizure request, either at handoff or at system access, not by falling C/I. Thus, a review of Borg reveals no disclosure or suggestion of a system or method such as that described and claimed herein.
In order to overcome the disadvantage of existing solutions, it would be advantageous to have a method in a digital or dual-mode radio telecommunications network for improving system performance by determining the expected C/I on a seized digital traffic channel prior to utilizing the channel, and, if the expected C/I is poor, reselecting a DTC or an AVC which will provide improved voice quality. The present invention provides such a method.